The present invention relates to processes for epitaxial growth of semiconductor layers.
Molecular beam epitaxy (MBE) is an extremely important technique whereby layers of semiconductors (or indeed of dielectrics or of metals) can be custom tailored with extremely sharp transitions between layers. This is useful not only in producing heterojunctions, such as between gallium arsenide and aluminum gallium arsenide, but can be used to produce a wide variety of exotic structures. Molecular beam epitaxy is an extremely versatile fabrication technique, and is essential to creation of a tremendous variety of inovative device structures.
A particularly important area of application of MBE techniques is in III-V device structures. Interfaces between III-V compound semiconductors are often of very good quality, and pairs of semiconductors exist within this family which have nearly perfect lattice match but very different band gaps (such as gallium arsenide and AlGaAs), so that this family of materials is extremely attractive for fabrication of various device structures which make use of heterojunctions or other applications of pseudo-potentials, i.e. of the changing potential energy profiles for a carrier which are produced by changes in the band gap of the material in a direction which the carrier might travel.
However, useful as MBE techniques are in research, fruition of their full promise has been prevented by the difficulties of growth-related surface defects. The present invention is aimed at reducing the defect density produced by prior art techniques for MBE growth of III-V epitaxial layers.
For molecular beam epitaxy growth generally, a very clean wafer is suspended facing downward in an ultrahigh vacuum chamber. The vacuum used is preferably 1E(-10) Torr or less. The wafer is typically held facing in a generally downward direction, to minimize particles falling on its surface, and two or more effusion cells (which are tiny furnaces with shutters, containing a pure source of one element) are heated and opened, so that atoms escaping the hot effusion source impinge on the surface of the substrate. The substrate is heated by a heater to a high enough temperature that atoms which impinge on its surface can move around to find an energetically favorable site. These energetically favorable sites will, for a clean crystalline wafer, be sites which correspond to the existing lattice, and thus epitaxial growth can occur.
In MBE growth of III-V compound semiconductors, it is strictly the flux of the group III element (e.g. aluminum, gallium, or indium) which limits the growth rate. This is because the vapor pressures of the group V elements (e.g. phosphorus or arsenic) are typically quite high at the temperatures required for epitaxial growth of crystalline semiconductor layers. Thus, to avoid outgassing of the group V element from the surface layer already in place on the wafer on which growth is proceeding, the vapor pressure at the group V element must be kept high enough to maintain equilibrium at the wafer's growth interface. Thus a very high density of group V atoms is continually impinging on the surface of the substrate, but, unless these atoms find a group V-deficient site (e.g. an adsorbed group III adatom), they will rapidly be desorbed from the wafer surface.
The present invention teaches that a major source of defects in MBE growth is deposition of cold group III atoms which have leaked around the shutter of the group III effusion cell before the substrate has been heated up to growth temperature. Thus, the present invention teaches that the substrate should be heated all (or nearly all) the way up to growth temperature before the group III effusion cell is fully heated. Thus, any group III atoms which leak out around the shutter of the group III effusion cell and thence impinge on the substrate surface will have enough mobility to find energetically favorable sites, i.e., sites which extend the lattice.
This problem of leakage becomes a particular difficulty when larger size wafers are used, because then, to achieve reasonably uniform rates of deposition of the group III element across the wafer surface, it will typically be necessary (for the same effusion cell type) to position the effusion cell farther from the wafer surface and run the cell at a higher temperature to provide the needed flux. However, the substantially higher temperature within the effusion cell means that leakage out around the shutter of the effusion cell is more likely.
It has not been recognized in the prior art that the sequence of heating the substrate before heating the group III effusion cell is of any criticality whatsoever. Nevertheless, the present invention teaches that this sequence of steps is critical, and that the density of defects in MBE-grown material is thereby reduced substantially. Moreover, this sequencing of steps is particularly desirable when the group III element in question is a metal which will be liquid at the growth temperature, since such low-temperature melting metals (indium and gallium) present particular problems with growth-related surface defect formation.
According to the present invention there is provided: A method for growing epitaxial layers consisting essentially of III-V semiconductor material, comprising the steps of: providing a crystalline substrate; suspending said substrate in an ultra-high vacuum facing a plurality of effusion cells; heating at least one of said effusion cells containing a group V element to provide a vapor pressure of said group V element at the surface of said substrate which is approximately in equilibrium with a group V component of the surface of said substrate at a predetermined growth temperature; heating said substrate to at least 2/3 of said growth temperature in degrees Kelvin; heating another one of said effusion cells containing a group III element to a predetermined growth effusion temperature; opening the shutter of said group III effusion cell while maintaining said substrate at said predetermined growth temperature, whereby epitaxial growth occurs on said substrate; wherein none of said effusion cells containing a group III element is heated hotter than 100 degrees C. below said growth effusion temperature before said substrate is heated.